1. Field of the Invention
The present invention relates to a photovoltaic electrochromic device.
2. Description of Related Art
What we call an electrochromic device is a device constituted of conductive materials for performing color changing when an electric field or current is applied to cause a reversible redox reaction. The fabrication of an electrochromic device should satisfy the following requirements: under different voltages, colors of the electrochromic device should be easily recognizable; the change of colors should be rapid and uniform; the reversibility of the color changing of the device should be repeatable for at least ten thousand times; and the device should have high stability. Commonly-used electrochromic devices are surface confined thin-film electrochromic devices and solution-type electrochromic devices.
A surface confined thin-film electrochromic device is formed by a top transparent substrate, a bottom transparent substrate, and an electrochromic multi-layer disposed therebetween. Specifically, the electrochromic multi-layer has a structure similar to a battery, which at least has five coated/deposited layers of different functions. The aforesaid five coated/deposited layers are a transparent conductive layer, an electrochromic layer, an electrolytic layer, an ion storage layer, and another transparent conductive layer. The solution-type electrochromic device has a simpler structure and is formed by a top transparent conductive layer and a bottom transparent conductive layer, which are bonded by an epoxy resin adhesive in a direction facing an electrode layer, and an electrochromic organic solution is disposed between the top and the bottom transparent conductive layers. The solution includes oxidation-type or reduction-type electrochromic organic molecules, a polymer electrolyte, and a solvent.
After years of research, only electrochromic rear-view mirrors have been commercialized. Other large-sized electrochromic devices 100 still face the problem of non-uniform color changing, which is also called an iris effect, as shown in FIG. 1. The explanation of the iris effect is based on FIG. 2. FIG. 2 illustrates a general electrochromic device 200, which is constituted of two transparent conductive substrates 210 and an electrochromic solution 220 disposed therebetween. When the electricity is provided from an electrode 230 disposed on the periphery of the two transparent conductive substrates 210, the difference in the path length of the electric fields in the center and the periphery of the plane electrochromic device 200, causes variation in the impedance; the difference in impedance is illustrated in FIG. 1. The gradual homocentric change of color concentration is displayed from the periphery to the center of the electrochromic device 200, which affects the uniformity of color changing.
To extend the application of electrochromic technology, researches that integrate photovoltaic technology have provided diverse directions for development. For instance, building integrated photovoltaic (BIPV) solar cells may be cooperated with the electrochromic technology to automatically adjust the colors of electrochromic windows to reduce indoor heat, based on indoor and outdoor illumination differences, which does not require any additional power supply. As power saving becomes more and more important, such an application has become a new trend.
For example, U.S. Pat. No. 5,377,037 has disclosed an electrochromic device which integrates silicon thin-film solar cells with electrochromic materials. In view of the structure thereof, silicon thin-film solar cells of a tandem structure, an electrochromic device, and an electrolytic layer are sequentially disposed between two transparent conductive glass substrates. Finally, bleed resistor are connected in series outside the two transparent conductive glass substrates of the tandem structure, so as to activate or deactivate the voltage which drives the electrochromic device when the silicon thin-film solar cells generate power. Although the double-side electrode structure can integrate the electrochromic device with solar cells, inorganic materials require greater charge density and larger voltage for performing color changing. Inevitably, the intrinsic layer needs to be thicker, so as to enhance the efficiency of photovoltaic conversion. Multi-junction stacked tandem cells may even be applied to increase the open circuit voltage (Voc) of the silicon thin-film solar cells. Consequently, the transmittance of the silicon thin-film solar cells is reduced.